1. Field of the Invention
The present invention relates to an actuator, and more specifically relates to a piezoelectric actuator suitable for use as an actuator of for example a hydraulic servo valve.
2. Description of the Prior Art
A linear motor has conventionally been used as the actuator for a high response hydraulic servo valve, wherein the valve spool is driven linearly by the linear motor. Further, in order to obtain a higher speed operation of the servo valve etc., a single stage magnification type piezoelectric actuator has already been proposed, incorporating a laminated piezoelectric element, and a displacement magnification mechanism which enlarges a very small displacement of a piezoelectric element by a lever effect. In order further to increase the output displacement of the actuator, and to make the driving of a driven member even faster, a piezoelectric actuator including a two-stage displacement magnification mechanism was proposed, as described in for example Japanese Utility Model Laying-Open Publication Sho 61-44080 (1986) or Japanese Utility Model Laying-Open Publication Sho 61-103679 (1986).
However, in the construction described in Japanese Utility Model Laying-Open Publication Sho 61-44080 (1986), the second stage of the displacement magnification is carried out hydraulically by using a nozzle and a flapper, and therefore there is the problem that the response is poor. Further, in the embodiments shown in FIG. 1 and FIG. 5 in that publication, two piezoelectric elements are used to operate in mutually opposite directions. Therefore, in that construction, unless the control signals to the two piezoelectric elements are controlled accurately and with an appropriate relative timing, the actuator will not operate appropriately and accurately. In the construction described in Japanese Utility Model Laying-Open Publication Sho 61-103679 (1986), in the first stage of displacement magnification mechanism, a lever construction is used in which a lever comes into contact with the fulcrum, and since the output displacement of the piezoelectric element is extremely small, and when the lever begins to pivot about the fulcrum there is a certain amount of play between the two, it is not possible for the first stage of the displacement magnification to be carried out accurately, and therefore it is difficult to ensure that the driven member is accurately driven and positioned.
In Japanese Utility Model Laying-Open Publication Sho 61-50448 (1986) there is described a print hammer in which the first stage displacement magnification mechanism is a lever mechanism with an elastic hinge as the fulcrum, and the second displacement magnification mechanism uses an elastic deformation of a spring; thus the printing pin can be driven at high speed according to an electrical control signal supplied to the piezoelectric element. In this print hammer, however, unless the leaf spring constant is increased, the resonance point is low, while if the leaf spring constant is increased, the lever arms of the first displacement magnification mechanism will be pushed apart, and therefore an accurate and smooth operation will no longer be obtained. Further, since the second stage of the displacement magnification is carried out by elastic deformation of a leaf spring, positioning of the driven member coupled to it cannot be carried out accurately, and therefore if this construction is applied to an actuator, the driving and positioning of the driven member cannot be carried out with both high accuracy and good response.